1. Field of the Invention
The present invention relates to a measuring apparatus, measuring method and image forming apparatus and, more particularly, to measurement of an amount of toner applied on an image carrying member of an image forming apparatus.
2. Description of the Related Art
In an electrophotographic image forming apparatus, even when image formation is made under the same conditions, the density of a formed image is erratic. This is due to the influence of variations of various image forming parameters such as those of an amount of electrical charge of toner, the sensitivity of a photosensitive member, and efficiency of transferring toner, and variations of environmental conditions such as temperature and humidity.
Hence, the density or height of a toner image developed on a photosensitive member or intermediate transferring member is detected, and various image forming parameters such as supply and a charging potential of toner, an amount of exposure light, and a developing bias are feedback-controlled based on the detection result.
For example, the invention of U.S. Pat. No. 2,956,487 detects a potential formed by an electrostatic latent image formed by exposure on a photosensitive member or the image density of a toner image obtained by developing the electrostatic latent image, compares the detection value with a reference value, and controls the image density in accordance with the comparison result. Also, the invention of U.S. Pat. No. 4,082,445 compares a difference between the amount of reflected light on a non-image region on a photosensitive member and that of a referential toner image with a reference value, and supplies toner in accordance with the comparison result.
FIG. 1 is a view showing a general method of measuring the amount of reflected light. A patch sensor 25 includes a light-emitting diode (LED) 25a which emits near infrared light as a light-emitting element, and a photodiode (PD) 25b as a photoreceptor, and measures the amount of reflected light of a referential toner image 26. In other words, the sensor 25 measures the amount of applied toner mainly using the amount of specular reflected light.
FIG. 2 is a graph showing the sensor output of a 530 spectrodensitometer available from X-Rite. As shown in FIG. 2, the amount of applied toner can be measured based on the sensor output within a density range from 0.6 to 0.8. However, a change in amount of reflected light with respect to a change in toner density is slight in a high density range. That is, it is difficult to accurately measure the amount of applied toner from the difference between the amounts of reflected light over the full density range.
Japanese Patent Laid-Open No. 2003-076129 discloses the invention which measures the amount of applied toner in a high density range by introducing polarized light. FIG. 3 is a view showing the arrangement of a patch sensor 25′ of Japanese Patent Laid-Open No. 2003-076129. The patch sensor 25′ includes PDs 25c and 25d and prisms 25e and 25f in addition to the LED 25a which emits near infrared light and the PD 25b. 
Light emitted by the LED 25a is separated by the prism 25e into components (S wave) which oscillate in a direction perpendicular to an incidence plane and components (P wave) which oscillate in a direction parallel to the incidence plane. The separated S wave enters the PD 25c, and the separated P wave strikes the referential toner image 26. The P wave which strikes the referential toner image 26 is diffused-reflected, and some components are converted into S wave components. The reflected light from the referential toner image 26 is separated into S and P waves by the prism 25f. The separated S wave enters the PD 25d, and the separated P wave enters the PD 25b. 
FIG. 4 is a graph showing the output (curve B) from the PD 25b, and the output (curve D) from the PD 25d. The amount of specular reflected light (P wave) represented by curve B is corrected by the amount of diffused light (S wave), thus obtaining the amount of reflected light (curve H) in which the influence of diffused reflection is removed. With this method, the amount of applied toner can be measured up to a density of about 1.0, but it is impossible to measure a higher density.
On the other hand, a method using a laser displacement sensor has also been proposed (for example, Japanese Patent Laid-Open No. 4-156479 and Japanese Patent Laid-Open No. 8-327331). FIGS. 5A and 5B are views showing a laser displacement sensor 24, and FIG. 6 is a graph showing the measurement result of the amount of applied toner by the laser displacement sensor 24.
The laser displacement sensor 24 can measure a change in height (thickness) of a laminated toner layer (see FIG. 5A). However, in a dot pattern or line pattern on a highlight range shown in FIG. 5B, toner layers become discontinuous. That is, as shown in FIG. 6, the amount of applied toner in a density range in which toner layers are continuous can be accurately measured. However, the amount of applied toner in a low density range in which toner layers become discontinuous cannot be accurately measured.
As described above, when the patch sensor is used, it is difficult to measure the amount of applied toner in a high density range, and when the laser displacement sensor is used, it is difficult to measure the amount of applied toner in a low density range. Therefore, in order to accurately measure the amount of applied toner over the full density range, both the patch sensor and laser displacement sensor are used, so that the patch sensor is used for a range other than the high density range, and the laser displacement sensor is used for the high density range. However, this results in increases in cost and size of an image forming apparatus.